typeahead-operator.ts

import { MonoTypeOperatorFunction, Observable } from 'rxjs';
import {
  debounceTime,
  distinctUntilChanged,
  filter,
  map
} from 'rxjs/operators';

interface ITypeaheadOperatorOptions {
  minLength: number;
  debounceTime: number;
  allowEmptyString?: boolean;
}

export function typeaheadOperator(
  options: ITypeaheadOperatorOptions
): MonoTypeOperatorFunction<string> {
  return (source): Observable<string> =>
    source.pipe(
      debounceTime(options.debounceTime),
      filter((value) => value !== null || value !== undefined),
      filter((value) => typeof value === 'string'),
      filter((value) => {
        if (value === '') {
          return options.allowEmptyString;
        }
        return value.length >= options.minLength;
      }),
      map((value) => value.toLowerCase()),
      distinctUntilChanged()
    );
}